Always set IV length for ciphers that use an IV

This fixes an issue where the IV length would not be set if the length
was equal to the recommended length. The issue shows up at least when an
IV of length 12 (which is returned by `t.iv_len()`) is used with the
AES256 CCM cipher, as OpenSSL defaults the IV length to 7 bytes [^1] and it
would not be correctly set to 12.

[^1]: https://wiki.openssl.org/index.php/EVP_Authenticated_Encryption_and_Decryption

Closes sfackler#2244.

openssl/src/symm.rs

@@ -626,10 +626,8 @@ impl Crypter {
 
         ctx.set_key_length(key.len())?;
 
-        if let (Some(iv), Some(iv_len)) = (iv, t.iv_len()) {
-            if iv.len() != iv_len {
-                ctx.set_iv_length(iv.len())?;
-            }
+        if let (Some(iv), Some(_iv_len)) = (iv, t.iv_len()) {
+            ctx.set_iv_length(iv.len())?;
         }
 
         f(&mut ctx, None, Some(key), iv)?;